Why can I not continue on Runescape?

Basically Im trying to enter Mort Myre, to go to Mor'ton, to continue with my slayer task. So I went up to the gate which tells me I need permission from Drezel. I went back to Drezel and He said to me "What Are you doing up here", then my character says the same. And he tells me he is guarding the coffin (or something). There is no option for me to talk to him about entering the swamp. I have definately completed Priest In Peril. So can anyone tell me what I haven't done for Drezel..Please Help!!